Although several statistically significant clinical and histopathologic prognostic indicators are known in breast cancer, the natural history of this disease in a significant number of patients indicates that there are many breast tumors whose behavior cannot be predicted by evaluation of these parameters. These morphologic criteria are insufficient with respect to predicting the doubling time or the invasive and/or metastatic potential of a given tumor. These factors however may be reflected by intracellular biochemical events which may be elucidated "in situ" by the use of immunocytochemical techniques which permit retrospective analysis of paraffin embedded tissues. This investigation intends to explore and attempt to define the possible prognostic significance of the immunohistochemical detection of an RNA tumor virus-related antigen in human breast cancer tissues. This antigen, which is specifically immunologically related to gp52, the major glycoprotein of the mouse mammary tumor virus, can be localized in routine diagnostic paraffin sections of approximately 50% randomly selected malignant breast tumors. It is not found in normal breast tissues, benign lesions or malignancies of a different histogenesis. Earlier investigations of the mouse mammary tumor model as well as recent immunohistochemical studies on human breast carcinomas suggest that the qualitative and/or quantitative detection of this antigen correlates with the aggressiveness of a given tumor and consequently with host survival. This study shall attempt to assess the significance of the detection of the gp52-related antigen in the original tumor of breast cancer patients who have been followed for at least l0 years or who have died within that period. The sections used for the immunohistochemical studies will be cut from paraffin blocks retrieved from the files of the Surgicial Pathology Division of Columbia Presbyterian Medical Center and affiliated hospitals. At least 500 cases, including an equal number of stage I and stage II patients, whose initial surgery was performed during 1970 and 1971 will comprise the initial study. The immunohistochemical data will be correlated with survival and pathologic, clinical and epidemiologic factors to determine the value of this antigen as a prognostic factor and useful adjunct in the clinical management of breast cancer patients.